Thymosin alpha 1 is a biological response modifier polypeptide originally isolated from bovine thymus tissue. Thymosin alpha 1 is presently being tested in a number of clinical trials for its ability to augment the capability or the rate of restoration of immune responses in cancer patients. Thymosin alpha 1 is an acidic polypeptide consisting of 28 amino acid residues with an acetyl group blocking the amino terminus. It has a molecular weight of 3108 daltons and an isoelectric point of 4.2. This project has investigated the activity of thymosin alpha 1 fragments in a thymectomized guinea pig MIF assay using peripheral blood lymphocytes and measuring their reactivity to PPD. A region of the amino acid sequence was found which was necessary for the biological activity of the thymosin alpha 1 fragment to be evident. Amino acids 15-20 were identified as the critical region and were determined to be the active site of the polypeptide for this particular biological effect. Modifications of the N-terminus of thymosin alpha 1 did not affect its biological activity. The amino acid sequence of this particular region of thymosin alpha 1 seems to be unique and may represent the important region of this thymic polypeptide for induction of the capacity of lymphocytes to produce lymphokines.